According to statistic data of the U.S. Department of Energy, windows typically contribute about 30 percent of overall building heating and cooling loads. Traditional windows are not optimized for energy saving. For example, in hot summer days too much sun radiation comes into a house through windows, causing great increases in the cost for air-conditioner cooling.
Solar heat gain coefficient (SHGC) is the key parameter to evaluate the energy efficiency of a window. Low SHGC windows on the current market are generally achieved by applying an invisible, spectrally selective low-emittance (low-E) coating between the panes of dual-glazed windows. In some southern states such as Texas and Georgia, building energy codes require windows to have a SHGC of 0.40 or less.
Low SHGC windows on the current market can reduce heat gain of house in summer, but also reduce beneficial heat gain from sun radiation in winter, thus the energy saving is not optimized in regions having both cold winters and hot summers. Therefore a window that may dynamically tune the transmission of sun radiation is of great global interest.
Throughout this application, the sun radiation (or solar spectrum) mainly refers to the sunlight in the wavelength range from about 300 nm to about 3 μm, which contains more than 95% of the total heat/energy of the sunlight.
There has been great interest in using variable light transmission glass or glazing to achieve electromagnetic radiation control.
Several types of chromogenic switchable glazing structures have been discovered using suspended particle devices, electrochromic effects, and certain types of liquid crystal. In general, these structures absorb or diffuse the incident light.
A polymer-dispersed liquid crystal (PDLC) has already been used for privacy-protection window in the high-end housing market. PDLC generally includes liquid crystal droplets dispersed in a polymer matrix, and works between a transparent state and an opaque scattering state depending on the refractive index match or mismatching between the liquid crystal and the polymer matrix. The opaque state may keep some heat and light out in the summer time but it also prevents the see-through view of people working in the office or staying at home. Additionally as the heat/light is scattered rather than reflected in the opaque state, some heat is still transmitted through in the summer time. The description of PDLC can be found in U.S. Pat. No. 4,994,204 (Doane et al.)
A variable light attenuating dichroic dye guest-host device as discussed in U.S. Pat. No. 6,239,778 (Palffy-Muhoray et al.) may be used for controlling the light transmission in an energy-saving window that may electronically tune the light transmission based on the different level of dye absorption. Because light is absorbed rather than reflected using this device, however, in hot summer days the temperature of the thin film may become very high and the heat may still penetrate inside the room. As a result, the dye may degrade and the reliability of the device may be an issue.
A glazing structure disclosed in U.S. Pat. No. 5,940,150 (Faris et al.) discusses an adjustable trans-reflective window that is primarily based on two layers of electrically switchable cholesteric liquid crystals (CLCs) or a combination of static cholesteric liquid crystal layers and electrically switchable liquid crystal retarder or TN cell. Their ability to reject or transmit light of one or two circularly polarized states depends on the electric stimulus applied.
All these solutions use electrical power to switch or tune the light transmission and the heat radiation. As a result, they all suffer additional cost associated with the electrical wiring in window installation.
It is highly desired to develop a smart window that selectively controls the transmission of sunlight radiation through a window structure at different times of the day and year so that thermal loading upon the heating and cooling systems of residential, commercial and industrial building environments can be minimized. It is also of a great importance that blocking the heat radiation does not have to impact the illuminating function of the sunlight and see-through view from the window. Most currently used technologies block the heat and the visible light equally. Consequently, in hot summer times, they may save energy for cooling but consume more energy for interior lighting. Furthermore, to minimize the installation cost, natural tuning that responds to temperature without the electrical wiring is of a benefit to the customer.
Thus, there is a need for a thermal tuning glazing structure that reflects sunlight when the temperature is high and transmits sunlight when the temperature is low, yet permitting a see-through view through the structure without using electrical power.